Interpenetrating polymer networks are described, for example, by L. H. Sperling in Interpenetrating Polymer Networks and Related Materials, Plenum Press, New York and London (1981). A full or true interpenetrating polymer network (IPN) is a material containing two polymers, each in network form, with the two polymers having been polymerized or vulcanized independently in the presence of each other to form two networks which are intertangled (interpenetrated) with each other. The IPNs may be formed in different manners, with the synthesis (polymerization) and/or cross-linking (vulcanization) of the two polymers being sequential or simultaneous. Another mode of IPN synthesis involves simultaneous coagulation and cross-linking of two latex polymers to form an interpenetrating elastomeric network.
One type of IPN system is illustrated in U.S. Pat. No. 4,302,553 of Frisch et al. IPNs of this sort invlove a blend of two different prepolymers cross-linked in independent processes and permanently entangled with one another. These IPNs are thermoset in character.
Semi- or pseudo-IPNs have only one cross-linked phase or network which is within a continuous unlinked polymer matrix phase. It is possible with certain solvent soluble resins to extract this non-cross-linked phase, whereas that is not possible for the true IPN. As a result, the true IPN systems must be cast since once the components are admixed and the polymer formation takes place, the interpenetrating networks cannot be separated. The single cross-linked network of the semi-IPNs allows these materials to retain thermoplastic character, although semi-IPNs with thermosetting properties are also possible.
U.S. Pat. Nos. 4,500,688 and 4,713,739 disclose silicone systems which are vulcanized within a thermoplastic matrix to form semi-interpenetrating polymer networks which are either pure silicone polymers or hybrids of a silicone polymer and a non-silicone (e.g., vinyl) polymer. These silicone semi-IPNs provide surface and dielectric properties which approach those of silicones without significantly decreasing the mechanical properties of the thermoplastic matrix in which they are formed. Thus, the matrix thermoplastic is essentially unaffected by the vulcanization of the silicone network. However, it would be desirable if silicone semi-IPNs could be formed which actually improve certain mechanical properties of the thermoplastic matrix, particularly tensile or compressive strength.